By default multiple RewriteCond's are treated using a logical AND, so...
Code:
RewriteCond %{REMOTE_ADDR} ^1\.2\.
RewriteCond %{REMOTE_ADDR} !^1\.2\.3\.4$
RewriteRule...
Does exactly what you want. It says If the IP starts with 1.2. and does not equal 1.2.3.4, then apply the rule